A. Field of Invention
This invention pertains to a method of making vascular prostheses from expanded PTFE (ePTFE) such as grafts and stent liners, said prostheses having an increased diameter, and a decreased wall thickness as compared to prior art grafts so that they can be used as a prosthesis in blood vessels having a relatively large diameter such as the aorta. More particularly, the invention pertains to a method of making a graft formed of at least one or more layers of ePTFE having an extremely thin wall yet high longitudinal tensile strength. A stent may be used in conjunction with the ePTFE layers, which allows the resulting prosthesis graft to be implanted without the use of a balloon or other graft expanding means.
B. Description of the Prior Art
Studies have shown tubes made of expandable polytetrafluorethylene (ePTFE) are ideally suited for various devices such as vascular prostheses. Vascular prostheses can be used to replace or repair blood vessels. Tubes made of ePTFE exhibit superior biocompatability, and can be made with a variety of diameters so that they can be implanted surgically.
Moreover, grafts of this type have high tensile strength in both the axial (or longitudinal) and radial direction so that the prostheses are very safe and do not dilate over time.
Grafts made of two layers of ePTFE or other plastic materials are well known in the prior art, illustrated by U.S. Pat. No. 5,800,512, PCT WO 98/31305 and other references.
Generally, tubes for prior art ePTFE prostheses have been made using the following steps:
a. A PTFE resin is compounded with a lubricant (preferably a petroleum distillate, such as naphtha); PA1 b. The compound is compacted under pressure; PA1 c. The compacted mass is extruded into a tube using a standard ram extrusion process to its predetermined working diameter; PA1 d. The tube is dried to remove the lubricant; PA1 e. The dried tube is stretched longitudinally by up to 1000%; PA1 f. The longitudinally stretched tube is sintered or cured at high temperature while its ends are fixed to insure that the tube does not shrink to its original length.
A problem with the process for manufacturing grafts in this manner is that there is a narrow range of reduction radios that produce acceptable results. The reduction ratio is the radio of the cross-sectional area of the compacted mass to the cross-sectional area of the extruded material. If the reduction ratio is too low, the product will not have adequate strength for use as an implant. If the reduction ratio is too high, the pressure in the extruder will exceed safe manufacturing limits.
Large diameter prostheses with a wall thickness similar to that of natural vessel can be produced according to the prior art, but the resulting product is very weak because of the low reduction ratio. However, there is a need for strong, large diameter materials for surgical repair of larger vessels, such as the aorta. Furthermore, large diameter prostheses with thinner walls, which have more acceptable reduction ratios, are very difficult to produce according to the prior art because the extruded material is too fragile to be handled during the drying, expansion and sintering stages. However, there is a need for such large diameter, thin walled material for use in creating stent grafts for endovascular repair of large diameter vessels. Moreover, small diameter, thin walled material cannot be produced by the prior art because of the high reduction ratio of this material. This material is needed for creating stent grafts for endovascular repair of smaller vessels, including the carotid, femoral and renal arteries.